Telephone line interface circuits, similar to that disclosed by V. V. Korsky in U.S. Pat. No. 4,103,112 titled "Telephone Line Circuit With Differential Loop Current Sensing and Compensation" have been widely used in combination with telephone switching facilities manufactured by the assignee. One problem with this circuit is that its return loss characteristic in a portion of the voice band is at variance with a return loss specification of a telephony standards setting organization known as the Local Area Telecommunications Authority Switching Systems General Requirements (LSSGR). The standard is simply defined in terms of passive electrical components. Particularly, the ac terminating characteristics of a line interface circuit, being connected to terminate a telephone line, are to be that of a 900 ohm resistor connected in series with a 2.16 micro farad capacitor. The characteristic determining structure in the line circuit as described in the previously mentioned patent is that of tip and ring transformer windings being connected in series with a direct current blocking capacitor, across the telephone line. The capacitive requirement is provided by the value of the capacitor and the resistance requirement is provided by a resistive termination of a secondary winding in the transformer.
This structure would be perfectly satisfactory if it were not for the presence of tip and ring feed resistors which function to provide a path for direct energizing current flow for operation of the telephone line. The tip and ring feed resistors are usually of a sum total resistance of about 400 ohms, but can be of greater of lesser value. This 400 ohm value strikes a compromise between reasonable length of serviceable telephone line and maximum line current in a short line. As is well known, the impedance of any capacitive element varies inversely with frequency. In the case of the line circuit, voice frequency signals of less than about 500 Hz are effectively split between two significantly conductive paths, one being the intended path provided by the capacitor and the other being the unintended path of the tip and ring feed resistors in series with the battery source.